Listeria monocytogenes infection in Biozzi mouse lines with high or low antibody responses, or with high (Hi/PHA) or low (Lo/PHA) responses to phytohemagglutinin.

Dependent and independent variables influencing natural and acquired resistance to Listeria monocytogenes in Biozzi mouse lines, genetically selected for their antibody responses, were analyzed. Variations in interline (IL) difference were shown to depend upon the inoculum dose, age, and sex of the mice used. Further, when IL differences were measured using the growth curves of L. monocytogenes, it appeared that LL mice were more resistant than HL mice, while the opposite was observed when IL differences were appreciated using the mortality rate. Attempts to analyze such apparently contradictory results showed that the predominant mechanism in LL mice was a higher natural bactericidal capacity of resident macrophages, which might be compensated for in HL mice by a higher ability to recruit blood-borne monocytes during the secondary, nonspecific phase of resistance, being reinforced and associated with a higher DTH reaction to L. monocytogenes antigen. A similar, higher antilisterial resistance was also observed in other Biozzi lines, genetically selected for their high in vitro CMI response to PHA as compared with the Lo/PHA line.